Generally, a magnetic memory storage device includes the following two component parts: a head pad and a rigid information disk. The head pad supports an element capable of reading or writing data magnetically on the information disk. The information disk itself embodies two basic components--specifically, a rigid substrate with a coating of magnetic media on its surface.
Today's market for rigid magnetic storage is well established and growing, with even greater advances being foreseen through the utilization of thin film media technology. Increased information densities, higher disk rotation speeds, and lower head flying heights not only afford greater efficiencies in data storage and retrieval, but also demand extremely tight tolerances to be held in the substrate specifications for flatness, rigidity at high rotational velocities, and surface texture. Therefore, the substrate must be produced with sufficient surface flatness and smoothness so that it can cope with the recent requirement for high density recording necessitated by the desire for increased information per unit of surface area. Where the product is designed for the high performance market, high capacity and rapid access characteristics are key requirements. Moreover, the current trend toward small disk drives and less powerful motors, particularly for the rapidly developing markets for slimline and portable drives, calls for thin, lightweight, rugged disks that have high functional densities and area capable of withstanding frequent takeoffs and landings with no deterioration in performance.
Research has been ongoing to discover materials which would satisfy these enhanced requirements. Glass substrates, specifically chemically tempered glass, have been used in the art. However, this material possesses a number of shortcomings which limit its utility. Recently, research has led to the development of glass-ceramic materials suitable for use as substrates in magnetic memory devices. These suitable glass-ceramic materials include glass-ceramics containing lithium disilicate, canasite, or fine grained spinel-type crystals.
An important step in the production of low cost substrates suitable for magnetic memory disks is cutting the materials into the desired shapes. As discussed above, it is necessary for the cutting method to produce a product which is flat and smooth. As is well known, the glass-ceramic substrates are used in the form of a thin wafer. The glass-ceramic wafers are obtained by slicing a mass of material with a cutting device.
A variety of saws for slicing brittle materials into wafers have been developed. For example, in the semi-conductor industry, it is well-known to use annular saws, i.e. I.D. saws; to produce silicon wafers. In this method, an internally bladed slicing machine is equipped with a wheel, which is a thin plate of stainless steel in annular form and having a thickness of a few hundred micrometers. Fine diamond particles of 40 to 60 .mu.m diameter are electrodeposited on and imbedded in the internal periphery of the annular plate to form a cutting blade. A single crystal of silicon is cut by putting it under adequate contacting force at the diamond blade of the annular plate, which is rotating at a high velocity, under tension in the radial direction. As a result, the diamond particles grind off the single crystal material to produce a wafer.
This method has several disadvantages. Firstly, there is likely to be a loss of material corresponding to the thickness of the cutting blade. Secondly, such a mechanical cutter is likely to form a warped wafer surface. Accordingly, subsequent finishing or lapping is required to improve the flatness of the wafer. Wire saws were also developed for cutting brittle materials. U.S. Pat. Nos. 3,831,576 and 3,841,297 to Mech disclose a wire saw for cutting brittle materials such as quartz and ceramics. The machine includes a web of wires defining a cutting area formed by winding a continuous strand of wire around a number of elongated spaced-apart pulleys. The material to be cut is mounted in a fixed position on a mounting apparatus and is moved by the mounting apparatus into the cutting area for engagement with the wires. A cutting mixture, i.e., a slurry containing fine particles of cutting material and a viscous carrier, is applied to the cutting area. Similar wire saws are also disclosed in U.S. Pat. No. 3,942,508 to Shimizu and U.S. Pat. No. 4,494,523 to Wells.
Conventional wire saws suffer from a number of disadvantages. The use of a web of wires can produced sliced wafers of variable and inconsistent thickness due to wandering of the individual wires as they proceed through the material. In addition, such wire movement produces wafers with an unsuitable curvature. Further, changes in the tension and speed of the wire produce inconsistencies in the surface of the wafer. Accordingly, expensive subsequent finishing work, such as lapping, is required to improve the flatness and smoothness of the wafer surface. This greatly increases the cost of the parts being produced and results in material loss. In addition, some waists produced may be curved to such a degree that lapping is unable to produce flat surfaces. Another disadvantage of wire saws is that low cutting speeds must be used for brittle materials. Lastly, wire saws have high operating costs, with the wire and slurry generally being the largest cost components.
The present invention is directed to overcoming these deficiencies.